Natural gas is transported from place-to-place via pipelines. It is desirable to know with accuracy the amount of gas flowing in the pipeline, and particular accuracy is demanded when the fluid is changing hands, or “custody transfer.” Even where custody transfer is not taking place, however, measurement accuracy is desirable, and in these situations, flow meters may be used.
Ultrasonic flow meters are one type of flow meter that may be used to measure the amount of fluid flowing in a pipeline. Ultrasonic flow meters have sufficient accuracy to be used in custody transfer. In an ultrasonic flow meter, acoustic signals are sent back and forth across the fluid stream to be measured. Based on parameters of received acoustic signals, the fluid flow velocity in the flow meter is determined. The volume of fluid flowing through the meter can be determined from determined flow velocities and the known cross-sectional area of the flow meter.
Various transducers are employed in conjunction with an ultrasonic flow meter to measure parameters of the fluid stream. For example, transducers may be employed to measure temperature, pressure, gas composition, density, relative density, calorific value, etc. of the fluid stream.
Accurate temperature measurement is important in flow metering systems to allow mass, volume correction and energy calculations to be performed. Speed of sound in a gas (e.g., natural gas) is dependent upon the gas composition, and the gas temperature and pressure. Inversely, temperature may be considered to be a function of the speed of sound in the gas, gas composition, and pressure. In an ultrasonic metering system equipped with pressure and gas composition transducers, the measured speed of sound (computed from the transit times of acoustic signals), the pressure and gas composition can be used to estimate temperature.